Among the common methods of molding vehicle interior materials such as car door trims, instrument panels and consoles, the hot stamping method is widely employed as the low-pressure compression molding characterized in that the molding cycle time is short, energy can be saved and no adhesive use is needed. The most typical of the conventional vehicle interior materials molded by the hot stamping molding method is one obtained by piling up a skin material, a foamed pad material and a core material in this order and integrally molding the pile according to the hot stamping molding method.
In recent years, there is a demand for improvements of human safety and comfortableness in feeling, etc. with respect to the above vehicle interior material comprising a core material and a skin material and, interposed therebetween, a foam layer composed of a foamed pad material for providing the skin material with cushioning properties. Thus, the skin material as a structural part of the interior material is required to possess cushioning properties with high flexibility.
A conventionally employed foamed pad material for hot stamping molding to be used in the vehicle interior material with cushioning properties is composed of a crosslinked resin foam whose principal component is polypropylene.
However, the above foamed pad material composed of the crosslinked foam cannot withstand compressive force at high temperatures during the hot stamping molding, so that cell shrinkage and rupture occur to thereby reduce the thickness of the foamed pad material, that is, cause unfavorable permanent set. Thus, after the molding, the skin material is likely to have poor flexibility and become hard with the result that the foamed pad material cannot be provided with cushioning properties with high flexibility.
The inventors studied the use as a foamed pad material of a polyolefin foam whose expansion ratio was increased and further a flexible polyethylene foam in order to improve the flexibility and cushioning properties of the skin material. However, the foamed pad materials composed of these foams without exception suffered from thickness decreases, namely, grave permanent set when molded according to the hot stamping molding method and hence entirely lacked practicability.
Proposals for overcoming the above thickness decrease and permanent set encountered with the pad material during the hot stamping molding were presented in, for example, Japanese Patent Laid-Open Publication No. 5(1993)-254035 and Japanese Patent Publication No. 6(1994)-78449.
However, the proposals of the above literature for overcoming the thickness decrease and permanent set of the pad material are applicable only to the foamed pad material which is composed of a resin whose principal component is polypropylene and which has expansion ratio of not greater than 15 for hot stamping molding in order to prevent the thickness decrease and permanent set. Hence, after the molding, the skin material still tends to lack flexibility and remain hard, so that cushioning properties with high flexibility have not yet been realized.
Therefore, it is desired to develop a foamed pad material for hot stamping molding which is excellent in heat weldability with core material forming resins and which not only exhibits excellent heat resistance, strain resistance and moldability at the time of hot stamping molding at which compression and drawing are conducted at high temperatures but also enables the molded skin material to possess cushioning properties with high flexibility.